Inherited defects in neuronal voltage-dependent calcium channel (VDCC) subunits lead to ataxia, epilepsy, migraines and neurodegeneration in both human patients and mice. The relationship between the structural perturbation, the ensuing cellular defect, and the severity of the disease remains undetermined. However, in many of these animal models, there is an altered complement of VDCC expressed at synapses. In this application we focus our attention upon a genetically engineered mouse (alpha1A knockout (ko/ko) devoid of functional P/Q -type VDCC. The alpha1A ko/ko mouse is ataxic, epileptic, and dystonic; however, the alpha1A heterozygote (ko/+) is asymptomatic. Our recent immunohistochemical and biochemical data demonstrate that alpha1B, the VDCC subunit that forms the N-type VDCC pore, has altered expression levels and spatial localization in the alpha1A ko/ko mice. As P/Q and N-type VDCC share a common pool of auxiliary subunits (alpha2/delta, beta, and gamma), we hypothesize that loss of alpha1A leads to perturbations in expression of auxiliary subunits, which, in turn, alter spatial and temporal expression of N-type VDCC. To test this hypothesis and further characterize altered alpha1B expression as it relates to defining the threshold and acquisition of disease, we will undertake four Specific Aims: 1: Compare and contrast N-type VDCC expression in control, alpha1A ko/ko and alpha1A ko/+ mice with respect to alpha1B, beta, and gamma isoform composition to determine the extent to which the N-type VDCC expressed in alpha1A ko/ko mice is structurally altered. 2: Identify the membranes and synaptic compartments occupied by alpha1B to determine if there is selective and differential localization of alpha1B in the hippocampus of diseased alpha1A ko/ko mice or human biopsy samples. 3: Identify the interval in development when alpha1B and functional N-type VDCC are altered in expression in alpha1A ko/ko mice to determine if the perturbation correlates with altered expression of other targeting VDCC subunits. 4: Characterize the pattern of expression of VDCC subunits in primary hippocampal cultures prepared from alpha1A ko/ko and alpha1A ko/+ mice to determine if the perturbation we observe in the intact animal can be reiterated and subsequently recovered in vitro by introducing cloned VDCC subunits into the cells This work may set a promising course towards using compensatory dysregularion of alpha1B as a diagnostic feature of heritable disorders that arise from human alpha1A mutations (including episodic ataxia type-2, spinocerebellar ataxia type-6, and human hemiplegic migraine) or other seizure disorders (such as temporal lobe epilepsy and hippocampal sclerosis).